Add a new valgrind suppression

Add a new valgrind suppression

To cover from this boost-related valgrind warning we can do nothing about
(and seems relatively harmless):

==25072== Syscall param sigaltstack(ss) points to uninitialised byte(s)
==25072==    at 0x585ABF7: sigaltstack (in /lib/libc-2.8.90.so)
==25072==    by 0x4E44D35: boost::detail::signal_handler::~signal_handler() (in /usr/lib/libboost_unit_test_framework.so.1.35.0)
==25072==    by 0x4E45560: boost::execution_monitor::catch_signals(boost::unit_test::callback0<int> const&) (in /usr/lib/libboost_unit_test_framework.so.1.35.0)
==25072==    by 0x4E4564B: boost::execution_monitor::execute(boost::unit_test::callback0<int> const&) (in /usr/lib/libboost_unit_test_framework.so.1.35.0)
==25072==    by 0x4E4C052: boost::unit_test::framework::run(unsigned long, bool) (in /usr/lib/libboost_unit_test_framework.so.1.35.0)
==25072==    by 0x4E5B023: boost::unit_test::unit_test_main(bool (*)(), int, char**) (in /usr/lib/libboost_unit_test_framework.so.1.35.0)
==25072==    by 0x41AFC3: main (in /home/alb/devel/integratech/switchit/mios/posixx/test/test_suite)
==25072==  Address 0x7feffea50 is on thread 1's stack
==25072==